A prerequisite for the safe forward movement of a vehicle is the reliable optimum adherence of the vehicle tires to the roadway.
Suddenly appearing slippery or wet surfaces affect this adherence considerably and are therefore frequent causes of accidents, often with grave consequences. The specific problem here is that the vehicle's driver often does not recognize the slippery or wet surface or recognizes it too late, and therefore quickly loses control of the vehicle.
To overcome this dangerous situation, different systems have been proposed which support the driver in estimating the condition of the roadway surface and/or initiate automatic accident prevention measures.
Thus, for example, German Patent Application DE 197 18 623 A1 describes a system in which the condition of the roadway surface is determined using radar beams. However, the problem with this approach is that the known method does not provide clear results, and does not ensure reliable determination of the roadway condition.
Another approach to the above-mentioned problem is to draw conclusions regarding the condition of the roadway surface, in particular the coefficient of friction between tires and roadway surface, on the basis of the deformation of the tire profile as the tire rolls on the roadway. To do so, however, a certain number of sensors must be installed in the tires, which raises considerable implementation problems due to the specific installation site (wearing parts, high accelerations, difficult contacting for signal transmission).
In German Patent Application DE 29 12 645 A1, a device for warning of a slippery surface is proposed, in which the condition of the roadway surface is determined using reflected electromagnetic waves at different frequencies. The device disclosed in the cited document, however, has considerable disadvantages. For example, the device described therein exhibits increased susceptibility to changing propagation conditions of the used electromagnetic signals. Changes in the propagation conditions may be caused by geometric changes, for example, short-term changes in the distance of the sensors from the roadway surface.